Did you ever give thought to the possibility that Frank might know what he's talking about? There are a lot of ways of nailing bank robbers, and you don't necessarily have a right to choose yours on the backs of innocent people. IFF you know for SURE that there is no backup AND you know for sure that you can kill this guy before he can kill or injure anyone else, I have no problem with your going for it. But what makes you so sure that that is the case? How do you know who each person in the bank is? Have you ever killed anyone before? How steady are you when people's lives depend on you? Speak from experience, please.
How about leaving the bank and nailing the guy when he gets outside? He's not going to let you? What makes you so sure he's going to let you shoot him inside the bank?
A lot of talk about a hypothetical that hasn't been fully hypothesized, and no one IN the situation would be likely to know what all the facts really are.
I don't trust much of what I read here, except maybe Frank's.